1. Field of the Invention
This invention relates to improved polymerization processes for the polymerization of lactams which are conducted in the presence of nitrothiophene and/or certain derivatives thereof which accelerate the polymerization. In a further aspect, this invention relates to accelerator compositions comprising such nitrothiophene compounds.
2. The Prior Art
Polylactams, such as poly-2-pyrrolidone (nylon-4) and polycaprolactam (nylon-6), are produced by the anionic (alkaline-catalyzed) polymerization of the lactam. The catalyst usually used comprises the reaction product of a lactam with an alkali metal, or quaternary ammonium hydroxide, or a source of alkali or alkaline earth metal, such as the hydroxide or alkoxide. The reaction product is generally recognized to be a lactamate, e.g., a salt, such as potassium pyrrolidonate, the product of the reaction between potassium and 2-pyrrolidone, having the formula: ##STR1## The salt consists of a cationic species such as K.sup.+, Na.sup.+, Ca.sup.++, N(CH.sub.3).sub.4.sup.+, etc., depending on the source of the catalyst, and an anionic species which can be a pyrrolidonate ion, a caprolactamate ion, etc. depending on the choice of lactam. Polymerization initiators and/or activators can be present during the polymerization reaction.
Various polymerization process have been suggested by the prior art, one of which is disclosed in U.S. Pat. No. 3,721,652, in which carbon dioxide is used as an activator for the polymerization of pyrrolidone. This patent also states that although it is preferable to use carbon dioxide as the sole activator, that other activators could also be used in combination with carbon dioxide.
As with most commercial processes, it would be desirable to increase the polymerization rate, thus reducing the size of the process equipment capacity and processing time required. One of the problems with polymerization accelerators is that they frequently cause an inferior, very low-molecular-weight polymer to be produced. Accordingly, it has now been discovered that by the use of certain nitrothiophene compounds, the rate of polymerization can be substantially increased, without substantially affecting polymer quality.